The present disclosure relates in general to a voltage-controlled oscillator (VCO) circuit. In particular, the present disclosure relates to a VCO circuit with broad tuning range and amplitude control.
A voltage-controlled oscillator is an oscillator that generates an output signal with a frequency that varies in response to an input control voltage. VCOs can be found in many circuit applications. For example, a VCO is the central block in phase-locked loops (PLLs) and as such is one of the most important blocks in data communication circuitry. Many circuit applications demand a wide output frequency range with stable amplitude for the VCO.
A VCO generates an AC output signal whose frequency changes in response to a control voltage signal applied at an input terminal.
The output frequency is inversely related to the period between alternating cycles of the output signal. A variable timing element is generally used to change the period between alternating cycles and thus change the output frequency. Typically, the variable element is either a variable capacitance diode (varactor) or a variable resistance device. Known devices in either category have intrinsic limitations with respect to thermal stability, linearity, adaptability to integrated circuit techniques, etc., that complicate the design of VCO circuitry.
A conventional LC-based VCO can be modeled with a RLC parallel network, in which a variable capacitor is used for frequency tuning. Due to the absence of a good varactor compatible with CMOS process, however, the tuning range of integrated CMOS LC-based VCOs is typically lower than 20% of the center frequency.
Since the lot-to-lot process variations in on-chip capacitance and inductance may be as large as 20%, the oscillation frequency may also have an error of 20% or more for lot-to-lot difference. The tuning range of a VCO must be wide enough to compensate for the fluctuation. In addition, amplitude variation is detrimental to circuit operation. A constant amplitude control loop is also incorporated in this circuit, ensuring stable oscillation amplitude over the whole tuning range.